1. Field of the Invention
The present invention is directed to a method for automatically setting an X-ray dosage for producing a tomographic image, using an X-ray CT device.
2. Description of the Prior Art
In the production of images of an examination subject in a computed tomography system, it is unavoidable that a certain level of noise will be present in the image. In general, the level of noise can be reduced by increasing the tube current, and thereby increasing the X-ray dose, however, obviously this cannot be done indiscriminately because overexposure of the examination subject to X-rays must be avoided.
Moreover, different portions of a human examination subject exhibit different attenuations, due to respectively different compositions and physical dimensions. Particularly in computed tomography examinations wherein a larger portion of the body is scanned, which may encompass such differently dimensioned body regions, it is undesirable, from the point of view of image quality, to select one and only one tube current/X-ray dose for the entire scan, because the dose may be suitable for scanning one portion of the subject, but may not be suitable for scanning another portion. If a single dose is employed for the entire scan, it must be selected at a compromise value.
Various investigations have been undertaken to analyze the relationship between the level of noise in a CT image, and the respective values of various parameters that are associated with the production of the CT image. One example of such an investigation is the article “Noise Due To Photon Counting Statistics In Computed X-Ray Tomography,” Chesler et al., Journal of Computed Assisted Tomography, Volume 1, No. 1 (1977), pages 64–74.